Species patch size at seeding affects diversity and productivity responses in establishing grasslands

Species interactions in diverse plant communities affect community-scale functions such as above-ground biomass production, diversity and invasion resistance. While the strength of these formative interactions can be affected by the balance of inter- and intraspecific interactions among the resident species, it is unclear over what distances individuals typically interact in grasslands and whether or not species interactions at seeding can be effectively manipulated to improve these responses. In a three-year study, we tested whether manipulating the size of seeded species patches affected above-ground biomass, diversity and invasion resistance in experimentally restored grassland plots (16m(2)). Plots were divided into patches that were 1, 0.5, 0.25 or 0.125m on a side, and the equivalent of 1m(2) in each plot was seeded with one of 16 grassland species (1728 seeds m(-2)). A final treatment involved mixing and broadcasting all seeds into an undivided plot to mimic typical restoration approaches. Using successively smaller initial seed patches resulted in plots that were less diverse and initially more productive (25% difference in second-year biomass production) than larger patch counterparts. Smaller patch plots also had a greater selection effect and experienced increases in resident species connectivity, suggesting that diversity declines and productivity gains resulted from the enhanced establishment and spread of more productive seeded species. Plots seeded with larger patches of the resident species were initially more invaded than those seeded with smaller patches, but this effect diminished over time. This likely reflects the ability of the non-seeded species to more effectively colonize larger patches with poor seeded species establishment. Mixed seeding plots were most similar to the smallest patch plots in their metrics of resident species spatial pattern, diversity and invasion resistance. However, these plots were initially less productive and had a weaker selection effect, which reflects a different compositional outcome, than the structured seeding plots.Synthesis. Species interact over sub-metre scales in establishing tallgrass prairie and, once established, their patterns may change over time. Given these dynamics, structured seeding approaches may be used over traditional mixed seeding approaches to control species dominance and preserve seeded species diversity within grassland systems.